Re: PATCH: (as177) Add class_device_unregister_wait() and platform_device_unregister_wait() to the driver model core

[Alan Stern <stern@rowland.harvard.edu>]

On Fri, 23 Jan 2004, Linus Torvalds wrote:

> On Fri, 23 Jan 2004, Alan Stern wrote:
> >
> > Since I haven't seen any progress towards implementing the 
> > class_device_unregister_wait() and platform_device_unregister_wait() 
> > functions, here is my attempt.
> 
> So why would this not deadlock?

Kind of a general question, so I'll give a general answer.  This wouldn't
deadlock for the same reason as anything else: People use it properly!

> The reason we don't wait on things like this is that it's basically
> impossible not to deadlock.

That's an exaggeration.  There are places where it's _necessary_ to
wait.  For example, consider this extract from a recent patch written by
Greg KH:

+	/* FIXME change this when the driver core gets the
+	 * class_device_unregister_wait() call */
+	init_completion(&bus->released);
 	class_device_unregister(&bus->class_dev);
+	wait_for_completion(&bus->released);

For the full patch, see 
http://marc.theaimsgroup.com/?l=linux-usb-devel&m=107109069106188&w=2

The general context is that a module is trying to unload, but it can't
until the release() callback for its device has finished.

> There are damn good reasons why the kernel uses reference counting 
> everywhere. Any other approach is broken.

And sometimes a part of the kernel has to wait until the reference count
drops to 0.  Instead of making everyone who needs to wait for a
class_device or a platform_device roll their own completions, this
provides a central facility.

By the way, adding class_device_unregister_wait() has an excellent
precedent.  The driver model core _already_ includes
device_unregister_wait(), merged by Pat Mochel.  Are you saying that
function shouldn't exist either?

Alan Stern